There's no direct comparison to trigger a warning; it's just seeing if a key in a dict exists.
And b'PYTHONCASEOK' is not a bug as that's required for OS X (remember this is technically posix.environ, not os.environ).

I think the best we can do is be platform-specific in what is checked for in the environ dict (e.g. b'PYTHONCASEOK' on darwin, 'PYTHONCASEOK' on win, and I don't know about cygwin), but otherwise one would have to iterate through the keys of the dict and do a type check before doing the comparison to fully avoid any potential warning being triggered.
Does the platform-specific decision of what to look for work for you, Serhiy?